1 Field
The invention relates to the field of geophysics.
More particularly, it provides a method of compensating birefringence in reflection seismic surveying.
2 Description Of The Related Art
Birefringence occurs each time a shear wave passes through an elastic domain that is affected by azimuthal anisotropy: the vibrations are then resolved along two perpendicular axes of anisotropy, with vibration S1 and vibration S2 on said two axes propagating at different speeds.
Each time a shear wave that was initially polarized with the polarization of one layer passes through a birefringent layer it becomes projected on the two polarization directions of the new layer.
In this way, the number of components in a signal is multiplied by two each time it passes through a layer having birefringent characteristics that differ from those of the preceding layer.
After passing through a plurality of such birefringent layers, the initial wave S is replaced by two sequences of waves, each polarized on the anisotropy axis of the last medium to be passed through, with each of these sequences comprising a sum of 2n−1 components, where n is the number of layer interfaces through which the wave has passed.
Thus, on leaving the last layer, a set of signal components is obtained in a first polarization together with a set of signal components in the second polarization.
In seismic applications, birefringency presents two opposing aspects.
It presents a positive aspect since it is theoretically possible to perform inversion for each layer and obtain the orientation of the anisotropy axes and the delay between slow propagation and fast propagation, and these parameters are of interest in exploring for oil (characterizing fractures).
It also presents a negative aspect since the complexity of the resulting sequence of 2n waves recorded by the seismic sensors confuses the final message. Birefringence needs to be inverted by calculation to recover the full potential of the exploration.
In the specification below, “dn” represents, for each layer n passed through, the time delay introduced between slow propagation S2 and fast propagation S1 (where n is an integer number corresponding to an index for the layers); “an” corresponds to the angle that exists between the fast axis of the layer n−1 and the fast axis of the layer n.
Algorithms are already known that make it possible to look for the parameters a and d relating to passing through a single layer.
These parameters a and d are generally calculated for trace portions having a duration of about 100 milliseconds (ms) (R. M. Alford, 1986, “Shear data in the presence of azimuthal anisotropy” SEG exp. abs., pp. 476-479; H. B. Lynn and Thomsen, 1990, “Reflection shear wave data collected near the principal axes of azimuthal anisotropy” Geophysics 55 (2), 147; L. A. Thomsen, I. Tsvankin, M. C. Mueller, 1995 “Layer stripping of azimuthal anisotropy from reflection shear wave data” SEG exp. abs., pp. 289-292; R. J. Garotta, “Detection of azimuthal anisotropy” 1989, SEG exp. abs., pp. 861-863).